U.S. patent application number 12/490081 was filed with the patent office on 2009-12-24 for image forming apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Keizo Takura.
Application Number | 20090317149 12/490081 |
Document ID | / |
Family ID | 41431443 |
Filed Date | 2009-12-24 |
United States Patent
Application |
20090317149 |
Kind Code |
A1 |
Takura; Keizo |
December 24, 2009 |
IMAGE FORMING APPARATUS
Abstract
An image forming apparatus includes a control unit configured to
cause an image forming unit to form an adjustment image on an image
bearing member, and a detection unit configured to detect the
adjustment image, wherein the control unit controls gradation for
forming an image by the image forming unit based on a detection
result provided by the detection unit, and wherein, when causing
the image forming unit to form a first adjustment image on the
image bearing member at a photosensitive member and to subsequently
form a second adjustment image that is different from the first
adjustment image, the control unit causes the image forming unit to
form the second adjustment image at a position different from a
position for forming the first adjustment image in a longitudinal
direction of the photosensitive member.
Inventors: |
Takura; Keizo; (Abiko-shi,
JP) |
Correspondence
Address: |
CANON U.S.A. INC. INTELLECTUAL PROPERTY DIVISION
15975 ALTON PARKWAY
IRVINE
CA
92618-3731
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41431443 |
Appl. No.: |
12/490081 |
Filed: |
June 23, 2009 |
Current U.S.
Class: |
399/301 |
Current CPC
Class: |
G03G 15/0131 20130101;
G03G 15/50 20130101; G03G 2215/00067 20130101 |
Class at
Publication: |
399/301 |
International
Class: |
G03G 15/01 20060101
G03G015/01 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 24, 2008 |
JP |
2008-165072 |
Jun 18, 2009 |
JP |
2009-145443 |
Claims
1. An image forming apparatus comprising: an image forming unit
including a photosensitive member, an exposure portion configured
to form a latent image on the photosensitive member, and a
developing portion configured to develop the latent image formed by
the exposure portion with toner to form the image developed by the
developing portion on an image bearing member, which is conveyed in
a direction orthogonal to a longitudinal direction of the
photosensitive member; a control unit configured to cause the image
forming unit to form an adjustment image on the image bearing
member; and a detection unit configured to detect the adjustment
image, wherein the control unit controls gradation for forming an
image by the image forming unit based on a detection result
provided by the detection unit, and wherein, when causing the image
forming unit to form a first adjustment image on the image bearing
member at the photosensitive member and to subsequently form a
second adjustment image that is different from the first adjustment
image, the control unit causes the image forming unit to form the
second adjustment image at a position different from a position for
forming the first adjustment image in the longitudinal direction of
the photosensitive member.
2. The image forming apparatus according to claim 1, wherein the
image forming unit includes a first photosensitive member and a
second photosensitive member, and wherein, when causing the image
forming unit to form the first adjustment image at the first
photosensitive member and a third adjustment image at the second
photosensitive member and to form the third adjustment image
subsequently to the formation of the first adjustment image in a
conveyance direction of the image bearing member, the control unit
causes the image forming unit to form the third adjustment image at
a position of the second photosensitive member corresponding to a
position in the longitudinal direction of the first photosensitive
member for forming the first adjustment image.
3. The image forming apparatus according to claim 2, wherein the
control unit causes the image forming unit to form the first
adjustment image and the second adjustment image at the first
photosensitive member and a third adjustment image and a fourth
adjustment image at the second photosensitive member, and wherein,
when causing the image forming unit to form the fourth adjustment
image and the second adjustment image side by side in the
conveyance direction of the image bearing member, the control unit
causes the image forming unit to form the second adjustment image
at a position of the first photosensitive member corresponding to a
position in the longitudinal direction of the second photosensitive
member for forming the fourth adjustment image.
4. The image forming apparatus according to claim 1, wherein the
image bearing member on which the first adjustment image is formed
and the image bearing member on which the second adjustment image
is formed are an identical recording medium.
5. The image forming apparatus according to claim 1, wherein the
first adjustment image is formed on a first recording medium, and
the second adjustment image is formed on a second recording medium
that is conveyed subsequently to the first recording medium.
6. The image forming apparatus according to claim 1, wherein the
image bearing member is a recording sheet, and wherein the control
unit causes the image forming unit to form the first adjustment
image on a position at an upstream side in a conveyance direction
of the recording sheet, to form the second adjustment image on a
position at a downstream side from the first adjustment image in
the conveyance direction of the recording sheet, and to form the
first adjustment image and the second adjustment image on positions
that are different from each other in a direction orthogonal to the
conveyance direction of the recording sheet.
7. The image forming apparatus according to claim 2, wherein the
image bearing member is a recording sheet, and wherein the control
unit causes the image forming unit to form the first adjustment
image on a position at an upstream side in a conveyance direction
of the recording sheet, to form the third adjustment image on a
position at a downstream side from the first adjustment image in
the conveyance direction of the recording sheet, and to form the
first adjustment image and the third adjustment image side by
side.
8. The image forming apparatus according to claim 7, wherein the
control unit causes the image forming unit to form a plurality of
adjustment images in a direction orthogonal to the conveyance
direction of the recording sheet and to form, in black, an
adjustment image located at a leading end among the plurality of
adjustment images.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an image forming apparatus
capable of optimizing gradation expression of input image data, for
example.
[0003] 2. Description of the Related Art
[0004] In an image forming apparatus such as a copying machine and
a printer using electrophotographic technology, an electrostatic
latent image is formed on a photosensitive member by uniformly
charging the photosensitive member by a charging roller and
exposing the photosensitive member to laser light, for example,
according to an image signal based on image data. The thus-formed
electrostatic latent image is developed with toner at a developing
portion, and the developed toner image is transferred onto a
transfer material by a transfer roller. The toner image transferred
onto the transfer material is fixed on the transfer material by a
fixing device, and then the transfer material is discharged from
the image forming apparatus.
[0005] In such an image forming apparatus, a high quality output
image is reproduced by selecting among various gradation expression
methods depending on the type of the image data (text/line-work,
graphic, map, developing paper, photograph, printing, etc.). To
stabilize the quality of an output image, the adjustment
(calibration) of image formation conditions such as density
correction and gradation correction is performed according to the
state of the image forming apparatus by forming a predetermined
pattern on an image bearing member in advance of the image
formation and reading a density of the predetermined pattern.
[0006] The adjustment is performed for the purpose of calibrating
minute fluctuations that are caused during the continuous use of
the image forming apparatus in reproducibility of gradation and
density of the gradually output image to standard and normal
levels. The fluctuations in image density and gradation
reproducibility includes a fluctuation due to a change in
environment and a fluctuation due to temporal changes of the
photosensitive member and the toner, and it is necessary to correct
these fluctuations at once to integrate the output image density
and the gradation reproducibility.
[0007] In the conventional method, a test pattern (test chart),
which is an index for correction, is firstly printed out on a
transfer material to find gradation characteristics of an output
image of the image forming apparatus itself. Subsequently, the
transfer material on which the test pattern is formed is placed on
a reader unit, and patterns of gradation levels are read by the
reader unit. After that, level values of the read-out gradations
and reference values previously stored in the image forming
apparatus are compared to each other, and, in the case where there
is a difference, the difference is fed back to adjust the image
processing conditions such as gradation correction to an optimum
state by which standard level printing is enabled.
[0008] In performing such calibration, an operator prints out the
test pattern on a transfer material for each of gradation
expression methods provided in the image forming apparatus. After
that, the work of placing (setting) the transfer material on the
reader unit for reading is performed for a number of times that is
the same as the number of gradation expression methods (e.g., for a
number of times that is the same as the number of transfer
materials on which the test patterns are printed) to perform the
adjustment for each of the gradation expression methods. Therefore,
the frequent calibration work may be bothersome for the operator,
and the number of transfer materials used for the calibration is
increased with a required time for the adjustment being increased.
In this regard, Japanese Patent Application Laid-Open No.
2003-054078 discusses a method for performing adjustment of image
processing conditions such as gradation correction based on test
patterns of two types of gradation expression methods that are
printed on one transfer material.
[0009] With the method, it is possible to reduce transfer material
consumption as well as to shorten the time required for adjustment
by printing the test patterns of two types of gradation expression
methods on one transfer material. However, since the test patterns
of different gradation expression methods are disposed adjacent to
each other in a sub-scanning direction, the test pattern to be used
for the adjustment is more subject to influences to be caused when
the gradation expression method is changed. The influences include
a memory image at the developing portion for developing the test
pattern, and a photosensitive member memory image on the
photosensitive member, formed when changing the gradation
expression method. Since the test pattern is used as the index for
the correction, once the test pattern is influenced by the
fluctuation attributable to the fluctuations in the image forming
apparatus and the fluctuations attributable to the changes in
photosensitive member and toner, the test pattern influences the
adjustment of the image processing conditions such as gradation
correction. More specifically, in the case where test patterns of a
plurality of gradation expression methods are printed on one
transfer material, influence of a memory image caused in formation
of a test pattern using one of the gradation expression methods
tends to be exerted on formation of a test pattern using another
one of the gradation expression methods.
[0010] In the case where the direction of a rotation axis of the
photosensitive member is set as the main scanning direction, a
direction orthogonal to the main scanning direction is referred to
as a sub-scanning direction, which is orthogonal to a rotation axis
of the transfer roller.
[0011] An memory image that may occur at the developing portion
will be briefly described below with reference to FIG. 16. For easy
understanding, an assumption of forming a pattern illustrated in
FIG. 16 on one recording medium P is made. The length direction of
the recording medium is a rotation direction of a photosensitive
member 4 and corresponds to a conveyance direction (direction of
the arrow) of the recording medium, and the width direction is an
axial direction of each of the photosensitive member 4 and a
developing cylinder provided in a developing portion 3 and
corresponds to a main scanning direction by a laser. In the example
illustrated in FIG. 16, a white hollow circle pattern is formed on
a solid black image (uniformly black image) on the left half part
in the axial direction of the photosensitive member 4 and the
developing cylinder, and a black hollow circle pattern is formed on
the right half part, followed by a halftone solid image (uniform
image), which is formed by changing the gradation expression
method.
[0012] In the developing portion 3 using electrophotographic
technology, a powdery developer called toner is housed in a toner
container inside the developing portion 3, and the toner is
uniformly coated on the developing cylinder, so that the toner is
conveyed to a nip portion between the developing cylinder and the
photosensitive member by the rotation of the developing cylinder.
During the conveyance of the toner by the developing cylinder, the
toner is electrically charged by friction with the developing
cylinder or friction between the toner particles, and an
electrostatic latent image formed on the photosensitive member is
developed as a toner image.
[0013] However, a part without an image is not developed by the
toner even when the toner is transferred by the developing
cylinder. Therefore, when the developing cylinder is rotated once
more, a difference in toner electrical charge amount can sometimes
occur on the developing cylinder between a part developed by the
first rotation and a part not developed. In such case, a memory
image at a rotation cycle of the developing cylinder is generated
as illustrated in FIG. 16. As is apparent from FIG. 16, memory
images of the solid black image, the white hollow circle pattern,
and the black hollow circle pattern are formed, due to influence of
the images that are formed previously, on the area on which the
halftone solid image is uniformly formed by changing the gradation
expression method. The memory images (abnormal images) may occur
not only in the second lap but also in the third lap of the
rotation of the developing cylinder as illustrated in FIG. 16, and
may also occur even in the fourth and fifth laps.
[0014] In the case where the abnormal image is generated on the
test pattern during the calibration, since cyclical irregularity
occurs on the test pattern due to the influence generated by
another gradation expression method, the image adjustment can be
unsuccessful.
[0015] Also, the photosensitive member memory image means a vaguely
remaining image of an image formed at a previous lap of the
rotation of the photosensitive member that is subjected to image
exposure depending on a state of the photosensitive member. The
photosensitive member memory image is substantially similar to that
illustrated in FIG. 16 except that the image is generated at the
photosensitive member cycle, and the formed test pattern is subject
to the influence of the photosensitive member cycle, thereby making
it difficult to perform the image adjustment successfully.
[0016] It is possible to reduce the number of recording mediums to
be used by printing test patterns of a plurality of gradation
expression methods on one recording medium. However, the influence
of the memory image generated due to the formation of the test
pattern of one of the gradation expression methods is exerted on
the test pattern of another one of the gradation expression methods
and further on the adjustment of the image processing conditions
such as the gradation correction.
[0017] The above example is described as a problem on one recording
medium since the influence is of the pattern formed at a leading
part of one recording medium. Also, in the case where a continuous
pattern is formed from a leading end to a trailing end in the
conveyance direction of the first recording medium, for example, a
problem of a memory image similar to that described above occurs on
a pattern to be formed on a second recording medium when an
interval between the recording mediums is short. In this case, it
is possible to alleviate the influence by widening the sheet feed
interval between the first recording medium and the second
recording medium for a predetermined multiple number of a
peripheral length of the developing cylinder. However, an extra
recording medium output time is required for the increase in sheet
feed interval.
SUMMARY OF THE INVENTION
[0018] The present invention is directed to an image forming
apparatus capable of reducing influence of a memory image in a
photosensitive member or a developing portion during
calibration.
[0019] According to an aspect of the present invention, an image
forming apparatus includes an image forming unit including a
photosensitive member, an exposure portion configured to form a
latent image on the photosensitive member, and a developing portion
configured to develop the latent image formed by the exposure
portion with toner to form the image developed by the developing
portion on an image bearing member, which is conveyed in a
direction orthogonal to a longitudinal direction of the
photosensitive member, a control unit configured to cause the image
forming unit to form an adjustment image on the image bearing
member, and a detection unit configured to detect the adjustment
image, wherein the control unit controls gradation for forming an
image by the image forming unit based on a detection result
provided by the detection unit, and wherein, when causing the image
forming unit to form a first adjustment image on the image bearing
member at the photosensitive member and to subsequently form a
second adjustment image that is different from the first adjustment
image, the control unit causes the image forming unit to form the
second adjustment image at a position different from a position for
forming the first adjustment image in the longitudinal direction of
the photosensitive member.
[0020] Further features and aspects of the present invention will
become apparent from the following detailed description of
exemplary embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate exemplary
embodiments, features, and aspects of the invention and, together
with the description, serve to explain the principles of the
invention.
[0022] FIG. 1 is a diagram illustrating an overall structure of a
color copying machine.
[0023] FIG. 2 is a perspective view illustrating a structure of a
printer engine unit.
[0024] FIG. 3 is a block diagram illustrating an image processing
unit.
[0025] FIG. 4 is a block diagram illustrating a printer control
unit.
[0026] FIG. 5 is a block diagram illustrating gradation correction
by a look-up table (LUT).
[0027] FIG. 6 is a diagram illustrating characteristics of steps
for reproducing an original image.
[0028] FIGS. 7A and 7B are flowcharts illustrating gradation
correction processing.
[0029] FIG. 8 is a diagram illustrating an example of a display
screen of an operation unit.
[0030] FIG. 9 is a diagram illustrating a test pattern image formed
of color gradation patch patterns.
[0031] FIG. 10 is a diagram illustrating an example of a
relationship between a laser output level used when a test print is
output and a density value obtained by reading the patches of the
output test print.
[0032] FIG. 11 is a diagram illustrating an example of a color test
pattern image in the case of arranging three patch groups of color
gradation patch pattern.
[0033] FIG. 12 is a diagram illustrating an example of a color test
pattern image in the case of arranging four patch groups of color
gradation patch pattern.
[0034] FIG. 13 is a diagram illustrating a color test pattern image
formed of color gradation patch patterns.
[0035] FIG. 14 is a diagram illustrating a combination of color
test pattern images in the case of continuously outputting a color
test pattern image over a plurality of recording sheets.
[0036] FIG. 15 is a diagram illustrating another combination of
color test pattern images in the case of continuously outputting a
color test pattern image over a plurality of recording sheets.
[0037] FIG. 16 is a diagram illustrating an example of a memory
image at a developing portion in an image forming apparatus using
electrophotographic technology.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0038] Various exemplary embodiments, features, and aspects of the
invention will be described in detail below with reference to the
drawings.
[0039] FIG. 1 is a sectional view illustrating a color image
forming apparatus 100 according to an exemplary embodiment of the
present invention. FIG. 2 is a perspective view illustrating a main
portion of a printer engine unit 300. The color image forming
apparatus 100 includes a reader unit 1 for reading an original
image and a printer unit 2 for reproducing (recording) an image on
a recording medium, which is an image bearing member, based on
image data obtained by the reader unit 1.
[0040] In the reader unit 1, a document 101 placed on a document
positioning plate 102 is irradiated by a light source 103. Light
reflected from the document 101 is focused on a charge-coupled
device (CCD) sensor 105 via an optical system 104. The CCD sensor
105 is provided with three lines of CCD line (array) sensors (not
illustrated) that are disposed adjacent to one another in three
lines, to which red (R), green (G), and blue (B) filters are
attached. Color component signals for red, green, and blue are
generated by the line sensors from the light made incident via the
optical system 104.
[0041] Also, the light source 103 and the optical system 104 scan,
as a document scanning unit, the document 101 while performing the
above-described operation and moving at a predetermined speed to
obtain the color component image signal of each of the lines of the
image in the document 101 by the CCD sensor 105. A reference white
board 106 is used for determining a white level of the CCD sensor
105 and performing shading correction in a thrust (array) direction
of the CCD sensor 105 and is disposed opposite the optical system
104. The shading correction is performed immediately before the
start of reading of the document 101 when the optical system 104
passes below the reference white board 106. An image signal output
from the CCD sensor 105 is subjected to predetermined image
processing by an image processing unit 130 and then input to a
printer control unit 140 of the printer unit 2.
[0042] An operation unit 120 is provided near the document
positioning plate 102, on which a switch for performing various
mode settings related to copy sequence of the color image forming
apparatus 100, a display for displaying, and a display unit are
disposed. Also, an instruction for staring operation of calibration
can be issued via the operation unit 120.
[0043] In the printer unit 2, the printer control unit 140, which
is a controller unit, includes a controller board provided with a
central processing unit (CPU), a random access memory (RAM), and a
read only memory (ROM). The color image forming apparatus 100
controls operations of a sheet feeding unit, an image forming unit,
a transferring/conveying unit, a fixing unit, and an operation unit
in an integrated manner based on a control program stored in the
ROM.
[0044] The printer engine unit 300 has the structure described
below. Photosensitive members 4a, 4b, 4c, and 4d each are supported
by a shaft at the center and rotatably driven by a driving motor (M
in FIG. 2) in the direction of the arrow. The four photosensitive
members 4a, 4b, 4c, and 4d are photosensitive drums for yellow,
magenta, cyan, and black, respectively. The photosensitive members
4a, 4b, 4c, and 4d are referred to as the first, second, third, and
fourth photosensitive members, respectively. Roller charging units
8a to 8d, a scanner unit 110, developing portions 3a to 3d,
cleaning devices 9a to 9d are disposed as opposed to an outer
periphery of the photosensitive members 4a to 4d and in a rotation
direction of the photosensitive members 4a to 4d. In the roller
charging units 8a to 8d, electrical charges having a uniform charge
amount are applied to surfaces of the photosensitive members 4a to
4d. Then, electrostatic latent images are formed on the
photosensitive members by exposing, by the scanner unit 110, the
photosensitive members 4a to 4d to light such as a laser beam that
is modified according to the recording image signal. Further, the
electrostatic latent images are developed by the developing
portions 3a to 3d, which contain developers (hereinafter, referred
to as "toners") of four colors of yellow, magenta, cyan, and black,
respectively.
[0045] Each of the developing portions 3a to 3d is used for
uniformly coating the toner on the developing cylinder and
conveying the toner to the nip portion of the photosensitive member
by the rotation of the developing cylinder. During the conveyance
by the developing cylinder, the toner is electrically charged by
the friction with the developing cylinder or the friction of toner
particles and then developed as a toner image on the photosensitive
member on which the electrostatic latent image is formed.
[0046] The thus-developed visible images are sequentially
transferred onto a recording medium (image bearing member) conveyed
by the transfer belt 5. After that, the residual toners on the
photosensitive members 4a to 4d are collected by cleaning devices
9a to 9d. By the above-described process, the image formations by
the toners are sequentially performed.
[0047] The sheet feeding unit includes a part for housing the
recording medium P, a portion for conveying the recording medium P,
a sensor for detecting passing of the recording medium P, a sensor
for detecting absence/presence of the recording medium P, and a
guide (not illustrated) for conveying the recording medium P along
the conveyance path. The recording medium P is housed in a cassette
15. A pickup roller 11 feeds the recording materials one after
another to convey the recording materials to the registration
roller 12.
[0048] The transfer/feeding unit will be described below in detail.
The transfer belt 5 has an electroconductive elastic layer formed
from a urethane rubber, silicon rubber, or a polychloroprene (CR)
rubber on a base layer, and a surface layer made from a fluorine
resin, or a fluorine-contained rubber (FKM) is formed on a surface
of the transfer belt 5. The transfer belt 5 is supported by a
driving roller 6 for transmitting the driving to the transfer belt
5, a tension roller for imparting an appropriate tensile force to
the transfer belt 5 by way of biasing by a spring (not
illustrated), and a driven roller 13.
[0049] The driving roller 6 is rotatably driven by a stepping motor
(not illustrated). Transfer rollers 10a to 10d are disposed at the
rear of the transfer belt 5 that is at a position opposed to the
photosensitive members 4a to 4d across the transfer belt 5.
Transfer rollers 10a to 10b apply a high voltage to transfer the
toner images to the recording material conveyed by the transfer
belt 5. Also, the transfer belt 5 is provided with a belt cleaning
device 14 for cleaning an image formation surface of the transfer
belt 5.
[0050] The fixing unit 7 is formed of a fixing roller provide with
an internal heat source such as a halogen heater and a pressing
roller (the roller is provided with a heat source in some cases)
which is pressed by the fixing roller.
[0051] When a print start signal is sent from a personal computer
connected to the color image formation apparatus 100 or the
operation unit 120, the recording medium P housed in the cassette
15 is conveyed by the pickup roller 11 one after another from the
top to the registration roller 12. Here, the registration roller 12
is stopped, and the tip of the recording medium P contacts the nip
portion.
[0052] When an image formation operation start signal is sent from
the printer control unit 140, electrostatic latent images are
formed on the photosensitive members for the colors by the exposure
units. The yellow electrostatic latent image is formed on the first
photosensitive member; the magenta electrostatic latent image is
formed on the second photosensitive member; the cyan electrostatic
latent image is formed on the third photosensitive member; and the
black electrostatic latent image is formed on the fourth
photosensitive member. The toner image formed on the photosensitive
member 4a (first photosensitive member), which is most upstream in
the rotation direction of the transfer belts 5, is transferred onto
the recording medium P, which is conveyed by the transfer belt 5,
by the transfer roller 10a, to which a high voltage is applied. The
recording medium P on which the toner image is transferred is
conveyed to a transfer area of the subsequent photosensitive
member.
[0053] At each of the image forming units, the image formation is
performed with a delay for a time period during which the toner
image is conveyed among the image forming units, so that the
subsequent toner image is transferred onto the recording medium P
with the leading end of the image aligned on the previous image.
Such process is repeated in the subsequent steps, so that the four
color toner images are transferred onto the recording medium P.
[0054] After that, the recording medium P is guided to the fixing
roller nip portion of the fixing unit 7. The toner images are fixed
on the surface of the recording medium P with heat of the fixing
unit 7 and a pressure of the nip. After that, the recording medium
P is discharged from the color image formation apparatus 100, so
that the series of image formation operations are terminated. In
the present exemplary embodiment, the image forming units are
disposed in the order of yellow, magenta, cyan, and black from the
upstream side, but the order is determined depending on performance
of the apparatus and is not limited to the example.
[0055] FIG. 3 is a block diagram illustrating an image processing
unit 130 according to the present exemplary embodiment. A ROM 216
in which a control program is written, and a RAM 215 storing data
for performing processing are connected to a CPU 214 via an address
bus and a data bus. Also, the CPU 214 is provided with an input
interface 250 for performing communication with external devices.
Also, an internal interface (I/F) unit 260 for performing
communication with the printer control unit 140 is connected to the
CPU 214. Control on the reader unit 1 including the following
structures is performed according to the program that is previously
stored in the ROM 216. The RAM 215 is used by the CPU 214 as a work
area, and a control program, and image processing parameters are
also stored in the ROM 216. The operation unit 120, which has a
keyboard (not illustrated), a touch panel (not illustrated), a
display unit 218 such as a liquid crystal display device, transmits
an instruction from an operator to the CPU 214 and performs display
of an operation mode and a state of the color copying machine under
the control of the CPU 214. The operation unit 120 is capable of
instructing the start of calibration.
[0056] An address counter 212 counts a clock CLK, which is
generated by a clock generation unit 211 at a unit of a pixel, to
output a main scanning address signal representing a pixel address
for one line. A decoder 213 decodes the main scanning address
signal output from the address counter 212. Simultaneously, the
decoder 213 outputs a signal 221 such as a shift pulse for driving
the CCD sensor by a unit of a line and a reset pulse, a signal VE
representing an effective interval in the signals for one line
output from the CCD sensor 105, and a line synchronization signal
HSYNC. The address counter 212 is cleared by the line
synchronization signal HSYNC output from the decoder 213 to start
counting of a main scanning address of the next line.
[0057] The RGB analog image signals output from the CCD sensor 105
are input to the analog signal processing unit 201 to adjust again
and offset. After that, conversion into RGB digital image data of 8
bits, for example, is performed on each of the color component by
an analog/digital (A/D) conversion unit 202. A line synchronization
signal HSYNC and a clock CLK on one-pixel-unit are added, at a
shading correction unit 203, to the RGB digital image data output
from the A/D conversion unit 202. Known shading correction is
performed on the RGB digital image data for each of the colors
using a signal obtained by reading a reference white board 106.
[0058] A line delay unit 204 corrects a spatial shift contained in
image data output from the shading correction unit 203. The spatial
shift is caused since the line sensors of RGB of the CCD sensor 105
are disposed with a predetermined distance being defined between
the adjacent line sensors in the sub-scanning direction. More
specifically, line delaying is performed in the sub-scanning
direction of the image data of each of color components R and G
based on the B color component signal to synchronize phases of the
three color component signals. A signal VE representing an
effective interval in signals for one line and a line
synchronization signal HSYNC are added to the RGB digital image
data.
[0059] An input masking unit 205 converts a color space of image
data output from the line delay unit 204 into an National
Television System Committee (NTSC) normal color space by a matrix
operation of the following expression (1). More specifically, each
of the color spaces of the color component signals output from the
CCD sensor 105 is determined depending on spectroscopic
characteristics of the filter of each of the color components, and
the color space is converted into an NTSC normal color space.
R0=a11 a12 a13 Ri
G0=a21 a22 a23 Gi
B0=a31 a32 a33 Bi (1)
where R0, G0, and B0 are output image signals, and Ri, Gi, and Bi
are input image signals.
[0060] In the case of using the color copying machine as a printer,
image data is input to the input interface 250 from an external
device such as a computer (not illustrated).
[0061] A LOG conversion unit 206 is formed of a look-up table
formed of a ROM, for example, and converts RGB luminance data
output from the input masking unit 205 into density data of C
(Cyan), M (Magenta), and Y (Yellow). A line delay memory 207 delays
the image signals output from the LOG conversion unit 206 for a
time period (line delay) during which a black character determining
unit (not illustrated) generates control signals such as UCR,
FILTER, and SEN from the outputs from the input masking unit
205.
[0062] The control signal UCR is used for controlling a masking/UCR
unit 208. The control signal FILTER is used by an output filter 210
for performing edge enhancement. The control signal SEN is used for
increasing resolution in the case where the black character
determining unit (not illustrated) determines a black
character.
[0063] The masking/UCR unit 208 extracts a black component signal K
from the image data output from the line delay memory 207. Further,
the masking/UCR unit 208 performs a matrix operation for correcting
color turbidity of the toners used as the developers of the printer
unit 2 on YMCK image data to output color component image data of 8
bits in a frame sequential manner of M, C, Y, and K, for example. A
matrix coefficient used for the matrix operation is set by the CPU
214.
[0064] A gamma correction unit 209 performs density correction on
the MCYK image data output from the masking/UCR unit 208 in a frame
sequential manner to adjust the image data to those having
gradation characteristics optimized for the printer unit 2.
[0065] The output filter (spatial filter processing unit) 210
performs edge enhancement or smoothing processing on the image data
output from the gamma correction unit 209 according to the control
signals from the CPU 214.
[0066] Also, a density conversion unit 220 is used for converting
the RGB image data output from the line delay unit 204 into data of
optical density.
[0067] The MCYK frame sequential color component image data
processed as described above is output to the printer control unit
140. Dither pattern image data expressed by a pseudo gradation
expression (gradation expression method) based on the type of the
image data is formed by the printer unit 2, and density recording
on a recording medium is performed based on a pulse signal output
based on the image data.
[0068] FIG. 4 is a block diagram illustrating the printer control
unit 140 according to the present exemplary embodiment. In the
printer engine unit 300 in FIG. 4, only one of the image forming
units for four colors is illustrated. Configurations of the image
forming units are basically similar although the operation timing
is varied in the rest of three image forming units.
[0069] The image data input from the image processing unit 130 of
the reader unit 1 to the printer control unit 140 is converted by a
dither circuit 26 into a pulse signal corresponding to the image
data. The pulse signal output from the dither circuit 26 is input
to a laser driver 27 to drive a laser light source of a scanner
unit 110 (exposure unit). The laser light output from the laser
light source based on the pulse signal from the dither circuit 26
becomes scanning light when reflected by a polygonal mirror (not
illustrated) rotating at a high speed. A path of the scanning light
is changed by the mirror to ultimately scan the photosensitive
member 4 in the main scanning direction, which is the axial
direction of the photosensitive member 4. Here, since the
photosensitive member 4 is being rotated in a direction indicated
by the arrow in FIG. 4 at a predetermined speed and is uniformly
charged by the roller charging unit 8, an electrostatic image is
formed on the photosensitive member 4 by the scanning of the
photosensitive member 4 by the laser light.
[0070] In each of the image forming units for the colors of YMCK,
the latent image is formed on the photosensitive member 4, and a
toner image is developed by the developing portion 3. In the
present exemplary embodiment, the two-component system is employed
as the developing method, and the image forming units are disposed
in the order of yellow, magenta, cyan, and black from the upstream
side in a direction along which the recording medium P is conveyed
by the transfer belt 5. Each of the image forming units forms an
electrostatic latent image on the photosensitive member 4 according
to the color to be reproduced under the control of the printer
control unit 140, and the electrostatic latent image is developed
into a toner image by the developing portion 3.
[0071] The recording medium P supplied from a recording sheet
cassette is conveyed and electrostatically attached to the transfer
belt 5. On the recording medium P conveyed by the transfer belt 5,
the toner image on the photosensitive member 4 is transferred at
the nip portion between the photosensitive member 4 for each of the
colors and the transfer roller 10. Accordingly, by a total of four
transfers, a toner image on which the four color toner images are
overlapped is formed on the recording medium P.
[0072] The recording medium P, on which the transfers of yellow,
magenta, cyan, and black are performed in this order, is detached
from the transfer belt 5, and then the toner image is fixed on the
recording medium P by the fixing unit 7, so that full color image
printing is accomplished.
[0073] In the printer control unit 140, a CPU 28 controls the
printer unit 2 including the printer engine unit 300 and the
following structures according to the program previously stored in
a ROM 30. Further, the CPU 28 communicates with the CPU 214 of the
reader unit 1 to perform operation such as copying in cooperation
with the CPU 214. A RAM 32 is used as a work area by the CPU 28,
and the ROM 30 stores control parameters in addition to the control
program. The RAM 32 includes a test pattern storage area 30a
(details of which are described below) in which data corresponding
to a predetermined test pattern is previously stored. Further, the
RAM 32 includes a backup area 32a, which is backed up by a battery,
to retain image formation parameters.
[0074] A look-up table (LUT) 25 is used for conforming the density
of a document image to the density of an output image. For example,
the look-up table 25 is formed of a RAM, and contents of the data
of the table are set by the CPU 28 in a calibration mode that is
started by an instruction from an operator via the operation unit
120 illustrated in FIG. 3. A pattern generator 29 outputs image
data for printing out a test print to the dither circuit 26 based
on data corresponding to the predetermined test pattern stored in
the test pattern storage area 30a in the calibration mode.
[0075] FIG. 5 is a block diagram illustrating gradation correction
by the LUT 25 according to the first exemplary embodiment of the
present invention.
[0076] The document luminance data output from the CCD sensor 105
is sequentially converted into density data by the image processing
unit 130 as described above. The density data is image data that
has been corrected based on gamma characteristics of the printer
unit 2 having the initial settings of factory default settings. The
image data output from the image processing unit 130 is input to
the LUT 25. The LUT 25 converts the density characteristics of the
image data input from the image processing unit 130 in such a
manner that the density of the document and the density of the
output image are identical to each other. The image data output
from the LUT 25 is input to the dither circuit 26.
[0077] Referring to FIG. 5, the printer unit 2 has a signal line
for inputting the image data read out by the reader unit 1 in the
case where the printer unit 2 is used as a copying machine. The
printer unit 2 as a printer may have two types of signal lines for
image data including a signal line for inputting image data from an
external device (PC 261). In the case where the printer unit 2 is
used as a copying machine, the image data read out by the reader
unit 1 is sent from the image processing unit 130 to the LUT 25
inside the printer unit 2. In the case of sending the image data
from the reader unit 1 to the LUT 25, the CPU 214 of the reader
unit 1 sends to the CPU 28 a signal for requesting start-up of an
image formation sequence of the printer unit 2 prior to sending the
image data.
[0078] In the case where the printer unit 2 is executing another
job when receiving the start-up request signal of the image
formation sequence from the reader unit 1, it is possible to reject
the request. Therefore, in the case where another job is being
executed, the CPU 214 of the reader unit 1 waits until an allowance
signal is sent from the CPU 28. The image data that has been
subjected to the gradation conversion by the LUT 25 is output as a
pulse signal corresponding to the image by the dither circuit 26 to
be sent to the laser driver 27, so that an electrostatic latent
image is formed on the photosensitive member 4.
[0079] FIG. 6 is a diagram illustrating characteristics of steps
for reproducing a document image by the color image forming
apparatus 100 according to the first exemplary embodiment of the
present invention.
[0080] In FIG. 6, the first quadrant indicates reading
characteristics of the reader unit 1, which converts the density of
a document image into a density signal. The second quadrant
indicates conversion characteristics of the LUT 25, which converts
the density characteristics of the density signal from the reader
unit 1. The third quadrant indicates recording characteristics of
the printer unit 2, which converts the laser output signal into
output density. The fourth quadrant indicates a relationship
between the document density of an original image and the density
of an output image by the printer unit 2 as well as gradation
reproduction characteristics of the color copying machine. The
number of gradations is 256 due to the 8-bit digital processing.
The document density and the density of an output image can be
measured by a commercially available density meter.
[0081] In the present exemplary embodiment, to make the gradation
reproduction characteristics shown in the fourth quadrant into
substantially linear characteristics, a non-linear part of the
recording characteristics of the printer unit 2 shown in the third
quadrant is corrected by the conversion characteristics of the LUT
25 in the second quadrant. The conversion characteristics of the
LUT 25 are set by an operation result.
[0082] Gradation correction control performed by the color image
forming apparatus will be described below. The gradation correction
control is performed in a calibration mode selected by the operator
via the operation unit 120.
[0083] FIGS. 7A and 7B are flowcharts illustrating gradation
correction processing according to the first exemplary embodiment
of the present invention. The processing is started when the
operator presses the start key 219 (FIG. 8) for the automatic
gradation correction (calibration) mode displayed on the display
unit 218 of the operation unit 120. The CPU 214 of the reader unit
1 and the CPU 28 of the printer unit 2 perform the gradation
control in cooperation. FIG. 7A illustrates a flowchart by the CPU
214 of the reader unit 1, and FIG. 7B illustrates a flowchart by
the CPU 28 of the printer unit 2.
[0084] In step S1 the CPU 214 of the reader unit 1 determines
whether the start key 219 is pressed by an operator. When it is
determined that the start key 219 is pressed (YES in step S1), then
in step S2, the CPU 214 starts up the pattern generator 29. Based
on data corresponding to a predetermined test pattern stored in the
test pattern storage area 30a, the CPU 214 generates the test
pattern (adjustment image) by using the pattern generator 29. The
CPU 214 gives an instruction to the CPU 28 for printing out an
image of the test print as illustrated in FIG. 9 from the printer
unit 2. Here, the LUT 25 is not used for outputting the test print.
In step S3, the CPU 214 displays on the display unit 218 a display
for prompting the operator to place the output test print on the
document positioning plate 102. In this case, the CPU 214 displays
a button to be pressed by the operator when the test print is
placed on the document positioning plate 102. In step S4, the CPU
214 determines whether the button is pressed by the operator, i.e.,
whether the document is set on the document positioning plate 102.
When the document is set on the document positioning plate 102 (YES
in step S4), then in step S5, the CPU 214 reads the test print by
using the CCD sensor 105.
[0085] FIG. 9 is a diagram illustrating an example of a test print
(image bearing member on which a test pattern (adjustment image) is
recorded) according to the first exemplary embodiment. Patches of
each of the colors (yellow, magenta, cyan, and black) have
gradations of three columns.times.12 rows (total 36 gradations),
and the test print is formed of patch groups of the four color
components of MCYK (total 144 patches). Here, the test print is
formed of patch groups (total 288 patches) of two different
gradation expression methods and has a first adjustment image and a
second adjustment image for each of the four colors of MCYK.
[0086] The patch groups of the two gradation expression methods
(dither screens) of the test print include a patch group (first
adjustment image) formed of a pattern of a high resolution dither
screen, which is the first gradation expression method, and a patch
group (second adjustment image) formed of a pattern of a low
resolution dither screen, which is the second gradation expression
method.
[0087] Further, the arrangement of density gradations in each of
the columns in each of the patch groups is such that the density is
increased from the upper end to the lower end of the recording
medium P. Here, the recording medium P is conveyed in a direction
from the upper end side to the lower end side. Also, the second,
low resolution dither pattern is disposed such that a density
arrangement in one color is asymmetrical to the color arrangement
of the identical color of the first, high resolution dither
pattern.
[0088] Patch position information of each of the patches of each of
the patch groups is numerically managed by the CPU 214 in the case
of exposing an image of the test pattern. In step S5, the CPU 214
reads the test print and calculates an arrangement of the test
pattern on the test print based on the column arrangement and the
arrangement of gradation density of the black patches recorded on
the test print for performing fine adjustment. The CPU 214 detects
light amount information of the recording sheet corresponding to
the ultimately determined pattern position data, i.e., light amount
information (R, G, and B values) of the test pattern recorded on
the test print.
[0089] To accurately perform the fine adjustment, the position of
an edge portion of one of the patch groups is accurately detected
by disposing the high density column of black at the end part of
the patch group in a direction orthogonal to the conveyance
direction of the recording medium P.
[0090] Here, if the test print illustrated in FIG. 9 is placed
upside down on the document positioning plate 102, the highest
density columns of black are allocated on an identical position.
However, since the gradation density arrangement is reversed, it is
possible for the CPU 214 to determine that the test print is placed
in a reverse direction and to automatically perform the
rearrangement of the light amount information of the test pattern,
thereby avoiding troubling the user.
[0091] Here, the CPU 214 performs control in such a manner that the
image signal is sent from the line delay unit 204 to the density
conversion unit 220. The CPU 214 previously sets a conversion
expression (table equivalent to the conversion expression) shown in
expression (2) in the density conversion unit 220, so that the
read-out RGB values are converted into optical density. The density
conversion unit 220 adjusts the conversion results with correction
coefficients km, kc, ky, and kk to obtain identical values with the
commercially available density meter. The base of the logarithm is
10.
M=-km.times.log(G/255)
C=-kc.times.log(R/255)
Y=-ky.times.log(B/255)
K=-kk.times.log(G/255) (2)
[0092] A reading point of the test pattern is set to a
substantially central area of each of the patches, and the CPU 214
calculates an average value of the read-out values. The CPU 214
converts the average read-out values (RGB signals) into YMCK
density values by the conversion expression (2) into the optical
density. In step S6, the CPU 214 performs the processing on all of
the patches in the test pattern. In step S7, the CPU 214 outputs
the processed values to the printer unit 2.
[0093] The CPU 28 of the printer unit 2 obtains gradation
characteristic information by the expression (2). More
specifically, the dither circuit 26 obtains the gradation
characteristic information of the laser output level that is set in
the laser driver 27 based on the actual density data (detection
result) of the test print obtained by the reader unit 1 and the
output from the pattern generator 29.
[0094] FIG. 10 is a diagram illustrating an example of a
relationship between a laser output level obtained when a test
print is output and a density value obtained by reading the patches
of the output test print according to the first exemplary
embodiment of the present invention. The horizontal axis indicates
the laser output level of the laser driver 27. The left vertical
axis is a density value obtained by reading an output image. The
right vertical axis is a density level of the output image, wherein
a density level when a base density value of a recording medium is
0.08 is set to "0", and a density value 1.60 is normalized to a
density level "255" as the highest density that can be output from
the color copying machine, for example.
[0095] Referring to FIG. 10, in the case where the density value of
the output image is particularly high as indicated by the point C
or low as indicated by the point D, the case wherein there is a
contaminant or scratch on a document positioning glass positioned
between the optical system 104 and the reference white board 106 or
the case wherein there is a defect in test print is assumed. In
such case, the CPU 28 performs correction by limiting an
inclination of a characteristic curvature so that continuity of
adjacent data sequences is stored. In the limitation, the
inclination is fixed to 3 when the actual inclination is 3 or more,
and a negative inclination is corrected to a value of a density
value of the previous one, for example.
[0096] In step S11, the CPU 28 of the printer unit 2 generates data
for a table to be set in the LUT 25 based on the gradation
characteristic information (characteristic curvature) illustrated
in FIG. 10 obtained in step S5. The table is set in the LUT 25 by
changing "the density level" of the right vertical axis to the
input side from the image processing unit (not illustrated) and
replacing "the laser output level" of the horizontal axis with the
output side to the dither circuit 26 in the gradation
characteristic curvature illustrated in FIG. 10. Such processing
means that the non-linear recording characteristic part of the
printer unit 2 shown in the third quadrant of FIG. 6 is corrected
by the conversion characteristics of the LUT 25 in the second
quadrant as described in the foregoing.
[0097] A density level that does not correspond to the patches is
calculated by an ordinary interpolation operation and smoothing
processing to be set as data of the table. Here, a restriction
condition of keeping an output level of "0" for an input level of
"0" is set.
[0098] In step S12, the CPU 28 sets the data of the table generated
in step S11 in the LUT 25.
[0099] In step S8, the CPU 214 of the reader unit 1 causes the
display unit 218 to display removal of the test print for which the
test pattern image reading is accomplished in step S12, so that the
operator removes the test print.
[0100] By the above-described processing, the gradation correction
processing in the calibration mode is terminated, thereby
accomplishing gradation correction excellent in gradation
reproducibility.
[0101] A part of the above-given description will hereinafter be
described in detail. FIG. 9 is a diagram illustrating an example of
the test print (recording sheet on which the test pattern is
recorded) according to the first exemplary embodiment. The test
pattern includes patches for gradations of 3 columns.times.12 rows
(total 36 gradations) for each of the colors (yellow, magenta,
cyan, and black), i.e., patch groups for the four color components
of YMCK (total 144 patches). The test pattern is formed of patch
groups for two different gradation expression methods (dither
screens) (total 288 patches). The color patches (for yellow,
magenta, cyan, and black) are formed by photosensitive members that
are provided for respective colors.
[0102] The patch groups for the two gradation expression methods
(dither screens) include a patch group formed of a pattern of a
high resolution dither screen, which is the first gradation
expression method, and a patch group formed of a pattern of a low
resolution dither screen, which is the second gradation expression
method. The patch group (first adjustment image) formed of the
pattern of the high resolution dither screen, which is the first
gradation expression method, and the patch group (second adjustment
image) formed of the pattern of the low resolution dither screen,
which is the second gradation expression method, are formed for
each of the colors.
[0103] In the color copying machine in the present exemplary
embodiment, the patch group formed of the pattern of the high
resolution dither screen, which is the first gradation expression
method, and the patch group formed of the pattern of the low
resolution dither screen, which is the second gradation expression
method, are positioned as described below. The pattern of the first
gradation expression method and the pattern of the second gradation
expression method to be formed by the same photosensitive member
are not allocated at an identical position in the longitudinal
direction of the photosensitive member or the developing sleeve
(axial direction in FIG. 9) at the upstream side and the downstream
side in the conveyance direction of the recording medium P. More
specifically, the patch groups of the identical color formed
according to the different gradation expression methods (first
adjustment image and second adjustment image) are not formed by
using the identical position in the longitudinal direction of the
photosensitive member or the developing sleeve (axial direction in
FIG. 9). Further, a density gradation arrangement in one column of
each of the patch groups is so arranged that the density is
increased from the upstream side to the downstream side in the
conveyance direction of the recording medium P. Also, the second,
low resolution dither pattern is arranged in a symmetrical fashion
with the first, high resolution dither pattern in terms of a color
order and a color density order.
[0104] A black patch group (first adjustment image) (black patch
group on the upper left side in FIG. 9) is formed with the high
resolution dither screen, which is the first gradation expression
method, by using the developing portion 3d and the photosensitive
member 4d (first photosensitive member). At the downstream side in
the conveyance direction of the recording medium P of the patch
group, a cyan patch group (third adjustment image) is formed with
the low resolution dither screen, which is the second gradation
expression method, by using the developing portion 3c and the
photosensitive member 4c (second photosensitive member), which are
different from the developing portion 3d and the photosensitive
member 4d. Here, the patch group is the cyan patch on the lower
left side in Fog. 9. The first color is black, and the first
gradation pattern is the black patch group (first adjustment image)
that is formed by using the first photosensitive member on the
upper left side in FIG. 9. The second color is cyan, and the third
gradation pattern is the cyan patch group (third adjustment image)
that is formed by using the second photosensitive member on the
lower left side in FIG. 9.
[0105] A yellow patch group (first adjustment image) is formed with
the high resolution dither screen, which is the first gradation
expression method, by using the developing portion 3a and the
photosensitive member 4a (first photosensitive member) At the
downstream side in the conveyance direction of the recording medium
P of the patch group, a magenta patch group (third adjustment
image) is formed with the low resolution dither screen, which is
the second gradation expression method, by using the developing
portion 3b and the photosensitive member 4b (second photosensitive
member), which are different from the developing portion 3a and the
photosensitive member 4a. Here, the first color is yellow, and the
first gradation pattern is the yellow patch group (first adjustment
image) that is formed by using the first photosensitive member and
positioned at second from the left in the upper column in FIG. 9.
The second color is magenta, and the third gradation pattern is the
magenta patch group (third adjustment image) that is formed by
using the second photosensitive member and positioned at second
from the left in the lower column.
[0106] A magenta patch group (first adjustment image) is formed
with the high resolution dither screen, which is the first
gradation expression method, by using the developing portion 3b and
the photosensitive member 4b (first photosensitive member) At the
downstream side in the conveyance direction of the recording medium
P of the patch group, a yellow patch group (third adjustment image)
is formed with the lowre solution dither screen, which is the
second gradation expression method, by using the developing portion
3a and the photosensitive member 4a, which are different from the
developing portion 3b and the photosensitive member 4b. Here, the
first color is magenta, and the first gradation pattern is the
magenta patch group (first adjustment image) that is formed by
using the first photosensitive member and positioned at third from
the left in the upper column in FIG. 9. The second color is yellow,
and the third gradation pattern is the yellow patch group (third
adjustment image) that is formed by using the second photosensitive
member and positioned at third from the left in the lower
column.
[0107] A cyan patch group (first adjustment image) is formed with
the high resolution dither screen, which is the first gradation
expression method, by using the developing portion 3c and the
photosensitive member 4c (first photosensitive member) At the
downstream side in the conveyance direction of the recording medium
P of the patch group, a black patch group (third adjustment image)
is formed with the low resolution dither screen, which is the
second gradation expression method, by using the developing portion
3d and the photosensitive member 4d, which are different from the
developing portion 3c and the photosensitive member 4c. Here, the
first color is cyan, and the first gradation pattern is the cyan
patch group (first adjustment image) that is formed by using the
first photosensitive member and positioned at fourth from the left
in the upper column in FIG. 9. The second color is black, and the
third gradation pattern is the black patch group (third adjustment
image) that is formed by using the second photosensitive member and
positioned at fourth from the left in the lower column.
[0108] Though the patch groups are described as the first
adjustment image and the third adjustment image in the above
description, the patch groups can also be used as the second
adjustment image and the fourth adjustment image. More
specifically, when the black patch group formed with the first
gradation expression method and by using the first photosensitive
member is the first adjustment image, the black patch group formed
with the second gradation expression method and by using the first
photosensitive member is the second adjustment image. In this case,
when the cyan patch group formed by the second gradation expression
method and by using the second photosensitive member is the third
adjustment image, the cyan patch group formed by the first
gradation expression method and by using the second photosensitive
member is the fourth adjustment image.
[0109] Also, when the yellow patch group formed with the first
gradation expression method is the first adjustment image, the
yellow patch group formed with the second gradation expression
method is the second adjustment image. In this case, when the
magenta patch group formed by the second gradation expression
method is the third adjustment image, the magenta patch group
formed by the first gradation expression method is the fourth
adjustment image.
[0110] Also, when the magenta patch group formed with the first
gradation expression method is the first adjustment image, the
magenta patch group formed with the second gradation expression
method is the second adjustment image. In this case, when the
yellow patch group formed by the second gradation expression method
is the third adjustment image, the yellow patch group formed by the
first gradation expression method is the fourth adjustment
image.
[0111] Also, when the cyan patch group formed with the first
gradation expression method is the first adjustment image, the cyan
patch group formed with the second gradation expression method is
the second adjustment image. In this case, when the black patch
group formed by the second gradation expression method is the third
adjustment image, the black patch group formed by the first
gradation expression method is the fourth adjustment image.
[0112] In other words, the patch group of the high resolution
dither screen, which is the first gradation expression method,
formed on the upstream side in the conveyance direction of the
recording medium P and the patch group of the low resolution dither
screen, which is the second gradation expression method, formed on
the downstream side in the conveyance direction of the recording
medium P are prepared. In this case, the patch groups are so
arranged that the patch groups of the identical color are not
allocated at the identical position in the longitudinal direction
of the photosensitive member or the developing sleeve (axial
direction in FIG. 9).
[0113] With such a configuration, the identical position in the
longitudinal direction on the developing portion 3 and the
photosensitive member 4 for one color is not used continuously for
the same color in the case of forming the patch groups of different
gradation expression methods. Therefore, it is possible to print
the patch group (second adjustment image) to be formed on the
downstream side in the conveyance direction of the recording medium
P with the various influences of the memory image otherwise caused
by the photosensitive member 4 and the developing portion 3 being
suppressed.
[0114] In the case where a certain color is successively arranged,
the following problems may occur. For the formation of the patch
group including a pattern of the low resolution dither screen,
which is the second gradation expression method, the photosensitive
member 4 and the developing portion 3 for one color that are used
at the upstream side in the conveyance direction of the recording
medium P that are used for forming the patch group of the first
gradation expression method are used at the identical position in
the longitudinal direction of the photosensitive member. Therefore,
various influences of memory image caused by the photosensitive
member 4 and the developing portion 3 may be exerted, but the
arrangement of the example is free from such influences and enables
printing the second patch groups for each of the colors.
[0115] The dither circuit 26 assigns 36 gradations in one patch
group including the pattern of the high resolution dither screen,
which is the first gradation expression method, among the entire
256 gradations of each of the patches based on the data output from
the pattern generator 29 (pattern generating unit). In this case,
the 36 gradations are mainly assigned to an area having low
density.
[0116] In contrast, the output level of the laser driver 27 is so
set that a smaller number of gradations are assigned to a high
density region. With such a configuration, it is possible to well
adjust the gradation characteristics particularly in a highlighted
portion (bright area).
[0117] On the other hand, the 36 gradations are uniformly assigned
to a reproducible density area level in the patch groups each
including the pattern of low resolution dither screen, which is the
second gradation expression method. However, it is necessary to set
the output level of the laser driver 27 in such a manner that finer
assignment is performed for a very low density area for the purpose
of accurate checking at the start of increase of density. With such
a configuration, it is possible to well adjust the gradation
characteristics in the entire reproducible density area of the
printer.
[0118] Also, in the color copying machine in the present exemplary
embodiment, characters and line images are formed by the high
resolution dither screen, while gradation images such as
photographs are formed by the low resolution dither screen, and the
gradation levels set for the test patterns are not necessarily be
identical.
[0119] As described above, it is possible to perform the gradation
correction by using the test pattern in the present exemplary
embodiment, and the use of the test pattern in the present
exemplary embodiment reduces the burden on the user in the case of
performing control for adjustment of density gradation of an
apparatus even when steps different from those of the present
exemplary embodiment are performed.
[0120] Also, though the test print in the present exemplary
embodiment has the three-column structure for the patch of each of
the colors, which is not more than an example, and the number of
columns may be 2, 4, or more. Further, though the patch of each of
the colors has 36 gradations (3 columns.times.12 rows), this is not
more than an example, too, and the number of gradations is not
limited to 36.
[0121] In the present exemplary embodiment, the patch groups of two
gradation expression methods (dither screens) are arranged on one
test print, but the gradation expression methods are not limited to
the different two types. Patch groups of three types of gradation
expression methods (dither screens) can be formed as illustrated in
FIG. 11, and patch groups of four types of gradation expression
methods (dither screens) can be formed as illustrated in FIG. 12,
or more gradation expression methods can be employed. The patch of
the gradation expression method formed on the upstream side in the
conveyance direction of the recording medium P and the patch of the
gradation expression method formed on the downstream side in the
conveyance direction of the recording medium P are so arranged that
an identical color is not allocated at an identical position in the
longitudinal direction of the photosensitive member or the
developing sleeve. With such a configuration, a similar effect can
be achieved.
[0122] Accordingly, it is possible to suppress the influences of a
memory image in the apparatus to be exerted on the test pattern in
performing calibration as well as to reduce the number of test
pattern sheets to be printed and discharged by the operator.
[0123] Further, when the color test pattern recorded on one
recording medium is divided into an upstream side and a downstream
side in the recording medium conveyance direction, the color
arrangements of the color test patterns of the upstream side and
the downstream side in the recording medium conveyance direction
are reversed with respect to the recording medium conveyance
direction. More specifically, as illustrated in FIG. 9, when the
order of black, yellow, magenta, and cyan from the left is set in
the upstream side, the order of black, yellow, magenta, and cyan
from the right is set in the downstream side. With such a
configuration, an algorithm for the apparatus to automatically
determine the direction of the test pattern is simplified, and an
algorithm for data rearrangement and calculation to be performed
afterwards is simultaneously simplified regardless of the direction
of the test pattern.
[0124] A second exemplary embodiment of the present invention based
on the image processing apparatus according to the first exemplary
embodiment will be described below. The image formation method and
the control method are the same as or similar to those of the first
exemplary embodiment, and a characteristic portion of a test
pattern enabled by the control for adjusting density gradation of
the apparatus will be mainly described.
[0125] The test pattern according to the present exemplary
embodiment is as illustrated in FIG. 13. The test pattern has a
color column arrangement different from that of the first exemplary
embodiment. High density gradation patch columns of black, yellow,
magenta, and cyan are firstly arranged, and lower density patch
columns are arranged sequentially in the same color order with
lowest density gradation patch columns being arranged lastly.
[0126] Alternatively, the arrangement illustrated in FIG. 14 can be
employed. The arrangement has the same color column arrangement
with the test pattern described with reference to FIG. 12. The
columns of close density levels of each of the colors are cyclical
but arranged at random.
[0127] With such a configuration, the gradation correction
processing in a wider area of the photosensitive member can be
performed. However, since a difference can occur due to gradation
shift at a connection part of patch columns, it is necessary to
optimize the smoothing processing at the connection part depending
on the apparatus.
[0128] Thus, according to the second exemplary embodiment, though
the difference due to the gradation shift may occur at the
connection part of patch columns as compared to the first exemplary
embodiment, it is possible to perform the gradation correction
processing on the entire plane, thereby enabling optimization of
the gradation of the entire plane.
[0129] A third exemplary embodiment of the present invention based
on the image processing apparatus according to the first and second
exemplary embodiments will be described below. The image formation
method and the control method are the same as those of the first
and second exemplary embodiments or based on a concept similar to
those of the first and second exemplary embodiments.
[0130] As illustrated in FIGS. 14 and 15, in the case of an image
forming apparatus wherein color test patterns are printed on a
plurality of recording sheets to be sequentially output, the image
forming apparatus has the structure of not allocating adjustment
patch images of an identical color at an identical position in the
recording medium conveyance direction. Here, patterns of a first
gradation expression and a second gradation expression are formed
on a first recording medium, and patterns of a third gradation
expression and a fourth gradation expression are formed on a second
recording medium. The patches of the gradation expression method
formed on the downstream side in the conveyance direction of the
first recording medium and the patches of the gradation expression
method formed on the upstream side in the conveyance direction of
the second recording medium are so arranged that an identical color
is not allocated at an identical position in the longitudinal
direction of the photosensitive member and the developing sleeve.
With such a configuration, it is possible to achieve a similar
effect.
[0131] The present exemplary embodiment is effective when it is
necessary to print many color test patterns and the test patterns
cannot be contained in one recording sheet. Also, the present
exemplary embodiment is effective in the case where there is a
combination of gradation expression methods that cannot be arranged
on one recording sheet due to a certain limitation of the printer
control unit.
[0132] With such a configuration, it is possible to suppress the
influences of the test pattern recorded on the first sheet to be
exerted on the second test pattern even when the sheet interval is
shortened.
[0133] Though the image bearing member is a recording medium, which
is a transfer sheet, in the foregoing description, the image
bearing member can be an intermediate transfer member. The above
exemplary embodiments are applicable to such case by changing the
detection unit to a sensor for reading a test pattern formed on the
intermediate transfer member.
[0134] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures, and functions.
[0135] This application claims priority from Japanese Patent
Applications No. 2008-165072 filed Jun. 24, 2008 and No.
2009-145443 filed Jun. 18, 2009, which are hereby incorporated by
reference herein in their entirety.
* * * * *